1. Field of the Invention
The present invention relates to a golf ball and more particularly to a golf ball having a cover layer made of synthetic resin.
2. Description of the Related Art
A flight distance is one of the important performances of the golf ball demanded by a golfer. The golf ball that flies a long distance refreshes the golfer and contributes to gaining good scores. It is necessary to improve the repulsive performance of the golf ball to improve the flight distance thereof.
Feeling is another important performance of the golf ball demanded by the golfer. The golf ball for which the golfer has a soft feeling gives the golfer a sense of security and contributes to the stability of swinging.
That is, it is important that the golf ball has a good repulsion performance and gives the golfer a soft feeling. To achieve this, various investigations have been made to improve the physical property of the golf ball. For example, in Japanese Patent Application Laid-Open No. 10-249, there is disclosed a golf ball having improved values in the physical property of the golf ball such as the hardness of the cover thereof, its bending elastic modulus, and the like. Other values of physical property of the golf ball such as its compressive strain amount, and Young""s modulus and loss factor, both of which are measured by a viscoelastic spectrometer have been also investigated.
The repulsion performance and the golfer""s feeling are manifested in the state in which the golf ball is actually hit, namely, in a dynamic state. On the other hand, the above-described hardness, bending""s elastic modulus, compressive strain amount, Young""s modulus, and loss factor are so-called static physical properties of the golf ball. Thus, no matter how much the static physical properties are investigated, it is difficult to achieve the improvement of the dynamic performance sufficiently.
A repulsive viscoelasticity spectrometer can be used as a means for measuring the physical property of the golf ball in the dynamic state. The repulsive viscoelasticity spectrometer measures the physical property of a specimen by applying a dynamic strain thereto. In Japanese Patent Application Laid-Open No. 8-141113, there is disclosed a golf ball having a cover whose Young""s modulus measured by the repulsive viscoelasticity spectrometer lies within a predetermined range.
The speed of the strain imparted to the material of the golf ball by the repulsive viscoelasticity spectrometer is as low as 0.001/sec to 0.1/sec, and a maximum strain is also as low as 0.01% to 2%. This is because the material of the golf ball has a high hardness and is thus not deformed greatly by a force applied thereto by the repulsive viscoelasticity spectrometer. On the other hand, when the golf ball is actually hit by a golfer, the speed of the strain of the golf ball lies in the range of 2000/sec to 5000/sec and its maximum strain is in the range as high as 5% to 25%. That is, the golf ball undergoes a high-speed and large deformation. There is a big difference between the degree of the strain of the golf ball in the case where a force is applied thereto by the repulsive viscoelasticity spectrometer and the degree of the strain thereof when it is actually hit. Therefore, the repulsive viscoelasticity spectrometer does not measure the dynamic property of the golf ball in a state similar to the state in which it is actually hit. To improve the repulsive performance of the golf ball and give the golfer a soft feeling when it is hit, it is necessary to optimize the dynamic property of the golf ball in a state similar to the state in which the golf ball is actually hit.
The present invention has been made in view of the above-described problems. Thus, it is an object of the present invention to provide a golf ball having an improved repulsion performance and giving a golfer a soft feeling when it is hit owing to its dynamic property optimized, based on measurement of the dynamic property in a state similar to the state in which the golf ball is actually hit.
To achieve the object, according to the present invention, there is provided a golf ball composed of a core layer and a cover layer, wherein the cover layer is made of a synthetic resinous composition whose Young""s modulus measured by a split Hopkinson""s bar tester lies in the range of 100 Mpa to 350 Mpa both inclusive and whose loss factor measured by the split Hopkinson""s bar tester lies in the range of 0.2 to 0.45 both inclusive, when an impact bar of the split Hopkinson""s bar tester collides with the golf ball at a speed of 14.0 m/sec.
The golf ball of the present invention has a cover layer made of a synthetic resinous composition. The Young""s modulus of the synthetic resinous composition and the loss factor thereof both measured by the split Hopkinson""s bar tester lie within a predetermined range. As will be described later, in measurement to be made by the split Hopkinson""s bar tester, a specimen undergoes a high-speed and large strain. Accordingly, the viscoelastic characteristic value (Young""s modulus and loss factor) of the specimen can be measured in a state similar to the state in which a golf ball is actually hit. It is possible to enhance the dynamic performance of the golf ball, namely, the repulsion performance thereof and feeling by optimizing the Young""s modulus and the loss factor thereof.
In the cover layer, the Young""s modulus of the cover layer measured by the split Hopkinson""s bar tester lies in the range of 100 Mpa to 350 Mpa both inclusive. Because the Young""s modulus lies within the range, the golf ball of the present invention has a preferable repulsion performance and gives a golfer a soft feeling when it is hit. That is, if the Young""s modulus of the cover layer is less than the range, the repulsion performance thereof may deteriorate, whereas if the Young""s modulus thereof is more than the range, the golf ball may give the golfer a hard feeling when it is hit. From this point of view, it is preferable that the Young""s modulus of the cover layer lies in the range of 150 Mpa to 250 Mpa.
The loss factor of the cover layer measured by the split Hopkinson""s bar tester (hereinafter may be referred to as merely xe2x80x9closs factorxe2x80x9d) is in the range of 0.2 to 0.45. Because the loss factor is in this range, the golf ball has a preferable repulsion performance and gives the golfer a soft feeling when it is hit. That is, if the loss factor of the cover layer is lower than the range, the the golf ball may be hard, which may give a bad feeling to the golfer when it is hit, whereas if the loss factor thereof is higher than the range, the repulsion performance of the golf ball may deteriorate. From this point of view, it is preferable that the loss factor of the cover layer lies in the range of 0.2 to 0.35 both inclusive.
The thickness of the cover layer is favorably in the range of 0.5 mm to 4.0 mm both inclusive and more favorably in the range of 1.2 mm to 3.0 mm both inclusive. When the thickness of the cover layer is in this range, the golf ball has a preferable repulsion performance and gives the golfer a soft feeling when it is hit.
The golf ball of the present invention has a core and a cover. The cover may consist of a single layer or a plurality of cover layers. When the cover has a plurality of cover layers, it is necessary that the Young""s modulus of the outermost cover layer and the loss factor thereof are in the above range. The cover may be a thread rubber-wound core or may be a solid core consisting of solid rubber. The solid core may be a single core layer or a plurality of core layers.
It is preferable that the core layer contains, as a main component thereof, polybutadiene containing a cis-1,4 linkage at 90% or more in a micro-structure thereof. Thereby, the golf ball has improved repulsion performance.